Tiny, monolithic accelerometers, pressure sensors, and other electromechanical transducers find application in a wide range of disciplines, from internal combustion engine automatic control systems to in vivo sensors for medical research to implantable hearing aids. In general, these transducers all have similar structures. A main body, which is relatively rigid, provides support for other elements of the transducer. Attached to the main body is a non-rigid or flexible structure which responds in some manner to an external stimulus applied to the transducer. For example, if the transducer is an absolute pressure sensor, the non-rigid structure may be a diaphragm covering an evacuated space in the main body. In order to detect the response, or strain, resulting from the external stimulus, or stress, strain sensing elements are attached to the non-rigid structure.
Usually, more than one of these elements is attached. This may be done either to provide a temperature reference or to increase the electrical output of the transducer. When four strain sensing elements are present, it is common to connect them in a Wheatstone bridge. For example, Nakamura U.S. Pat. No. 3,968,466 describes a miniature monolithic pressure transducer which has a circular diaphragm of monocrystalline silicon. Four piezoresistive strain sensing elements are formed in the diaphragm, oriented along the crystal axes so as to provide maximum sensitivity to strain. The four piezoresistors are, as shown in FIG. 6 of Nakamura, connected in a Wheatstone bridge.
The strain sensing elements of Nakamura and many of the other transducers operate on the principle of piezoresistance. That is, theyundergo a change in resistivity when mechanically deformed. Their resistivity is thus a measure of the amount of strain which the piezoresistors are experiencing. One such piezoresistive material is silicon. When it is desired to sense strain in a member, silicon piezoresistors may be deposited directly on the member, or, if the member itself is made of silicon, the piezoresistors may be formed integrally with it by one of the recognized methods, such as diffusion or ion implantation. The strain sensing elements described in Nakamura were formed in this way.
There are several obstacles to accurate mechanical-to-electrical conversion in transducers such as that described in Nakamura. One is the well-known dependence of semiconductor resistivity on temperature. Since the resistance of the strain sensing elements is to be used as a measure of strain, variations in that resistance due to factors other than strain, such as temperature, must be avoided. The Nakamura device attempts to accomplish this by placing all of the piezoresistors near one another. Another method of compensating for temperature is disclosed in Whitehead U.S. Pat. No. 4,141,253, which describes a force transducing cantilever beam. Six piezoresistors are diffused into the surface of the silicon beam, which is the non-rigid or flexing structure. The six resistors, connected in a Wheatstone bridge, perform different functions. Two are used to sense strain, two are dummy resistors, and the remaining two "are used for temperature compensation purposes." Although it is more accurate to compensate for temperature by means of dedicated piezoresistors, as done in Whitehead, than by simply grouping all of the strain sensing elements in a small area, the Whitehead arrangement still does not give acceptable results. One reason is that the temperature measuring resistors are formed in the cantilever beam along with the strain sensing resistors; both sets therefore measure strain as well as temperature. Since the output of this transducer includes both strain related and temperature related terms, temperature compensation must be accomplished externally, by a separate circuit. These calculations are complicated and the circuitry necessary to perform them is complex. The other reason for lack of acceptable results from the Whitehead transducer is due to the orientation of the temperature sensing resistors. While the strain sensing resistors are oriented along a direction of maximum strain, the temperature sensors are given a slightly different orientation in order to provide a mathematical basis for substracting the temperature terms. This off-axis orientation leads to inaccuracy in sensing along the desired axis, as will be described below.
An improved method of temperature compensation is described in Roylance, "A Miniature Integrated Circuit Accelerometer For Biomedical Applications," dissertation submitted to Department of Electrical Engineering, Stanford University, Nov., 1977. There, the strain sensing element is diffused into the surface of the cantilever beam, while the temperature-sensing element is diffused into a portion of the main body. The resistivity of the latter sensor, therefore, depends only upon temperature and not upon the strain of the beam. Even this arrangement, however, does not completely avoid the difficulties of temperature compensation. Roylance connects the two piezoresistors as adjacent arms in a Wheatstone bridge in which the remaining two arms are ordinary resistors. The output voltage of this transducer therefore includes both strain dependent and temperature dependent terms; and the temperature dependent terms must be removed by complex external circuits.
Another source of inaccuracy in these transducers is, as mentioned above in connection with the Whitehead reference, the possibility of measuring off-axis forces or accelerations. Since the electrical output of the transducer is indicative only of the magnitude of the acceleration, other factors must be considered in order to determine its direction. These factors are the orientation of the transducer itself, the directions of the crystal axes within the transducer body (for semiconductor transducers), and the orientation of the strain sensing elements. For any particular transducer construction, there will be a principal axis such that forces or accelerations along that axis are manifested more strongly than those along other axes. For the cantilever beam, for example, this principal axis is normal to the surface of the beam. If it is desired to measure accelerations in a particular direction, the transducer must be positioned so that its principal axis is aligned in that direction, and the strain sensing elements on the transducer must oriented along the direction of maximum strain of the transducer's non-rigid or flexing structure. Any variation in either the attitude of the transducer or the orientation of the piezoresistors will result in detection of an acceleration in a direction other than the desired direction. Conversely, if it is desired to measure an acceleration of unknown direction, a transducer constructed with a view toward measuring acceleration along only its principal axis will only indicate that component of the true acceleration.
One accelerometer design which purports to cancel off-axis acceleration measurements is described in U.S. Air Force Technical Report AFAL-TR-79-1175, Cantilever Accelerometer (Dec., 1979). In this accelerometer, eight piezoresistive elements are located on the flexible portion of the device, with four bing oriented so that their resistance increases with acceleration and the other four so that their resistance decreases with acceleration. The interconnection among these elements is a "dual-transverse bridge." The orientation and interconnection of the piezoresistors, together with the use of a symmetric proof mass (a large mass which responds to applied accelerations) is said to allow for cross-axis insensitivity.
Another factor contributing to below optimum performance of the prior art accelerometers described here is lack of uniformity among strain sensing elements formed on the same transducer. For best results, at any given level of stress, these elements should be as nearly equal as possible in both overall resistance and "gauge factor," which is the fractional change in resistance at specified stress. The gauge factor is a measure of the transducer's sensitivity. There are essentially two ways in which sensitivity can be increased: increasing the gauge factor or increasing the current through (and therefore the voltage across) the piezoresistors. Current, of course, is limited by the capacity of the device to dissipate heat.